Magnetic Forces and Fields

Magnetic Forces and Fields Law of Magnetism: Like magnetic poles repel and unlike magnetic poles attract • Magnets must have 2 poles – break a magnet in half …… • First magnetic mineral called lodestone • Gilbert (know his name) – Earth is a giant piece of lodestone • Gilbert responsible for early field ideas

Magnetic Forces and Fields Magnetic Field strengths – unit Tesla (T) You’ll learn more conversions later Relative Magnetic Field strengths: (table 12.1)

Magnetic Forces and Fields Concept Check, page 586 Magnetic Fields Gravitational Fields Electric Fields attraction and repulsion attraction only attraction and repulsion Towards or away from towards centre of towards or away poles on source source from centre of source act on other magnets act on all act on charged and ferromagnetic masses masses metals (Fe, Ni, Co)

Magnetic Forces and Fields Direction of magnetic field (by definition): The direction of force on the north-seeking pole of a compass or bar magnet N S N S N S N S N S

Magnetic Forces and Fields The pole marked “N” on a bar magnet is not a pole like the Earth’s north pole – it is the opposite. It is North-seeking. If the Earth was a bar magnet, our north pole would be stamped S!!!!!!!

Magnetic Forces and Fields Field around a bar magnet: To visualize, picture what a compass needle (or bar magnet) would do if placed near the bar magnet

Magnetic Forces and Fields Oersted, 1820 – magnetic field near a current-carrying wire: Demo Symbol: wire carrying current out of page wire carrying current into page How to remember: Think arrow

Magnetic Forces and Fields Using the first left hand rule given on page 588 (below) you determine the field around wires with electron flow out of and into the page

Magnetic Forces and Fields Field around an electromagnetic coil or solenoid given by second left hand rule. Another way of looking at it – do cross-sectional cut-away of coil. Like a bar magnet!

Magnetic Forces and Fields 12.2 Moving Charges and Magnetic Fields Second left hand rule: magnetic field line 00

Magnetic Forces and Fields Applications of electromagnets: relay switches, motors, alarm bells, door bells, speakers What causes magnets to be magnets? Domain theory: tiny magnetic fields generated in ferromagnetic materials by electron movement, add up to produce small regions of magnetic fields called domains. Domains are tiny (< 1 mm across)

Magnetic Forces and Fields Domains are scattered in the absence of an external magnetic field In the presence of an external field they line up, and the ferromagnetic material is attracted to a magnet, and behaves, itself, as a magnet Remove external field: they scatter again – if…………………..

Magnetic Forces and Fields Read Magnetism in Nature, page 590 and Then, Now, and Future, page 591

Magnetic Forces and Fields Check and Reflect, page 592 Discuss questions 4, 6, 7, 9, and 12

Magnetic Forces and Fields 12.2 Moving Charges and Magnetic Fields Experiments with Cathode Ray Tubes (CRT) – late 1800’s, led to many changes in atomic physics (last unit of Physics 30) A very simple cathode ray tube could be constructed as shown below: Other CRT’s – next page glass tube at near vacuum anode cathode + -

Magnetic Forces and Fields 12.2 Moving Charges and Magnetic Fields Thomson’s e/m

Magnetic Forces and Fields 12.2 Moving Charges and Magnetic Fields Beam seemed to come from the (-) terminal of the tube (cathode) – later substantiated Interaction of a moving charge with magnetic field: Don’t copy – just for explanation

Magnetic Forces and Fields 12.2 Moving Charges and Magnetic Fields Magnetic force is always perpendicular to the charged particle’s velocity In this course magnetic field also always perpendicular to velocity (except in descriptive situations) so force will be perpendicular to both velocity and magnetic field direction The 3rd Left Hand Rule can be used to determine the direction of the force on free charges and currents in a magnetic field

Magnetic Forces and Fields 12.2 Moving Charges and Magnetic Fields (or free negative charge) Go back to previous diagram to see that they are consistent

Magnetic Forces and Fields 12.3 Current Carrying Conductors and Magnetic Fields

Magnetic Forces and Fields 12.2 Moving Charges and Magnetic Fields

Magnetic Forces and Fields 12.2 Moving Charges and Magnetic Fields Use left hand for negative particles like electrons or anions; right hand for positive particles like protons, alpha particles, and cations Examples: Skills Practice, page 595 Cathode Ray Tube Demo a) Out of page b) Into page c) Into page (RH)

Magnetic Forces and Fields 12.2 Moving Charges and Magnetic Fields front of arc Figure 12.19, page 597

Magnetic Forces and Fields 12.2 Moving Charges and Magnetic Fields Northern (and Southern) Lights – Aurora Borealis and Aurora Australis Are the particles shown in the picture (+) or (-)?

Magnetic Forces and Fields 12.2 Moving Charges and Magnetic Fields Magnetic force calculations: Examples: Practice Problems 1, 3 page 599

Magnetic Forces and Fields 12.2 Moving Charges and Magnetic Fields Practice Problem 1, page 599 Magnitude only

Magnetic Forces and Fields 12.2 Moving Charges and Magnetic Fields Practice Problem 3, page 599 (fingers) direction at equator (thumb) (palm) down towards earth’s surface)

Magnetic Forces and Fields 12.2 Moving Charges and Magnetic Fields Do Check and Reflect, page 601, questions 3 and 5 Do SNAP Problems, page 120, questions 1, 3, 4, 6, and 10

Magnetic Forces and Fields 12.3 Current Carrying Conductors and Magnetic Fields Electric Current, I Unit Ampere (A) – commonly called amp

Magnetic Forces and Fields 12.3 Current Carrying Conductors and Magnetic Fields Force on a wire in a magnetic field similar to force on a free charge in a magnetic field Same hand rule Formula almost the same Free charge: Current in Wire: Where I = current (A) and = length of perpendicular wire in field (m)

Magnetic Forces and Fields 12.3 Current Carrying Conductors and Magnetic Fields New breakdown for 1 T, Earlier: Now: Don’t need to memorize these – show why not – but should be able to show that they’re equal

Magnetic Forces and Fields 12.3 Current Carrying Conductors and Magnetic Fields Example: Practice Problem 1, page 605

Magnetic Forces and Fields 12.3 Current Carrying Conductors and Magnetic Fields Example: Practice Problem 2, page 605 To find the answer you need a force equal and opposite to the force of gravity Do SNAP page 120, questions 2, 8 Review question 12

Magnetic Forces and Fields 12.3 Current Carrying Conductors and Magnetic Fields The magnetic field due to coil is either (2nd LHR) Since only wires carrying current perpendicular to field are affected, this means only section XY is involved or What direction is current in the coil? Hint: 2nd and 3rd LHR’s

Magnetic Forces and Fields 12.3 Current Carrying Conductors and Magnetic Fields Force between two current carrying wires: attraction N S repulsion N S Principle: same direction – attract opposite direction - repel

Magnetic Forces and Fields 12.3 Current Carrying Conductors and Magnetic Fields SI definition of the Ampere – current in each of 2 parallel wires, 1 m long, 1 m apart, that results in a force of 2.00 x 10-7 N Why isn’t 1 C/s good enough for defining the amp?

Magnetic Forces and Fields 12.3 Current Carrying Conductors and Magnetic Fields Concept Check page 608 – discuss Motors Use 3rd left hand rule to try to understand why armature moves and why commutator is necessary

Magnetic Forces and Fields 12.3 Current Carrying Conductors and Magnetic Fields Current must reverse direction No net force

Magnetic Forces and Fields 12.3 Current Carrying Conductors and Magnetic Fields Faraday and Henry’s independent discovery: electromagnetic induction Text diagram page 615 Faraday’s device – when switch was closed momentary current in one direction produced current then dropped to zero when switch was opened momentary current in opposite direction produced

Magnetic Forces and Fields 12.3 Current Carrying Conductors and Magnetic Fields Their Interpretation: Changing magnetic field across a wire produces current in that wire Changing magnetic field can be produced by: • movement of wire with respect to field • movement of field with respect to wire • varying size of the magnetic field with respect to wire

Magnetic Forces and Fields 12.3 Current Carrying Conductors and Magnetic Fields The classic generator generates electric current by the first method

Magnetic Forces and Fields 12.3 Current Carrying Conductors and Magnetic Fields Read pages 614 and 615 – new and old technologies Read An Accidental Discovery, page 617

Magnetic Forces and Fields 12.3 Current Carrying Conductors and Magnetic Fields Lenz’s Law: The direction of a magnetically induced current is such as to oppose the cause of the current. Demonstrations

Magnetic Forces and Fields 12.3 Current Carrying Conductors and Magnetic Fields N S Current is induced around walls of tube with a magnetic field that opposes the falling magnet Which way does the current flow? Hint: 2nd Left Hand Rule What happens as the magnet continues to fall?

Magnetic Forces and Fields 12.3 Current Carrying Conductors and Magnetic Fields Note that the current above the magnet has reversed to attract it back up while current below is continuing to repel it upwards N S

Magnetic Forces and Fields 12.3 Current Carrying Conductors and Magnetic Fields Eddy currents set up in face of paddle create magnetic field attracting them to poles of permanent magnet S N Why does it stop?

Magnetic Forces and Fields 12.3 Current Carrying Conductors and Magnetic Fields Check and Reflect, page 620, questions 1 – 7 Discuss questions from SNAP, pages 125 - 127

Magnetic Forces and Fields 12.3 Current Carrying Conductors and Magnetic Fields What direction will current flow if the conducting rod is dragged through the magnetic field as shown? According to Lenz’s Law there must be a force to left since rod is being pulled right. Point palm left (for force), fingers into the page for magnetic field. Your thumb points to bottom of page for current direction in the rod. In completed circuit current flow is clockwise.

Magnetic Forces and Fields 12.3 Current Carrying Conductors and Magnetic Fields If magnet moves, current will flow in coil to produce magnetic field to oppose its motion Move to left: right end of coil must become S (seeking) to repel. Use 2nd left hand rule to determine direction of current in coil and through galvanometer Move to right: right end of coil must become N (seeking) to attract. Current moves left through galvanometer. Current moves right through galvanometer

Magnetic Forces and Fields 12.3 Current Carrying Conductors and Magnetic Fields Lenz’s Law provides another way (I think easier way) to predict the current direction for a generator. Since the current produced opposes the motion that created it, the force (palm) goes against the wires motion, the magnetic field is fixed (fingers), and your thumb indicates the direction the electron flow current.

Magnetic Forces and Fields 12.3 Current Carrying Conductors and Magnetic Fields same electron flow (thumb) Lenz force (palm)

Magnetic Forces and Fields 12.3 Current Carrying Conductors and Magnetic Fields